Carbon dioxide, hydrogen sulfide and various hydrocarbons, such as methane, ethane, propane, normal butane and isobutane, are present in natural gas and other petroleum fluids. However, water is typically found mixed in varying amounts with such petroleum fluids constituents. Under conditions of elevated pressure and reduced temperature clathrate hydrates can form when such petroleum fluids constituents or other hydrate formers are mixed with water. Clathrate hydrates are water crystals which form a cage-like structure around guest molecules such as hydrate forming hydrocarbons or gases. Some hydrate forming hydrocarbons include, but are not limited to, methane, ethane, propane, isobutane, butane, neopentane, ethylene, propylene, isobutylene, cyclopropane, cyclobutane, cyclopentane, cyclohexane and benzene. Some hydrate forming gases include but are not limited to oxygen, nitrogen, hydrogen sulfide, carbon dioxide, sulfur dioxide and chlorine.
Gas hydrate crystals or gas hydrates are a class of clathrate hydrates of particular interest to the petroleum industry because of the pipeline blockages that they can produce during the production and transport of natural gas and other petroleum fluids. For example, ethane, at a pressure of about 1 MPa, can form gas hydrates with water at temperatures below 4.degree. C., while at a pressure of 3 MPa it can form gas hydrates with water at temperatures below 14.degree. C. Such temperatures and pressures are not uncommon for many operating environments where natural gas and other petroleum fluids are produced and transported.
As gas hydrates agglomerate they can produce hydrate blockages in the pipe or conduit used to produce and/or transport natural gas or other petroleum fluid. The formation of such hydrate blockages can lead to a shutdown in production and thus substantial financial losses. Furthermore, restarting a shutdown facility, particularly an offshore production or transport facility, is difficult because significant amounts of time, energy, and materials, as well as various engineering adjustments, are required to remove the hydrate blockage.
A variety of measures have been used by the oil and gas industry to prevent the formation of hydrate blockages in oil or gas streams. Such measures include removing free water, reducing the water content of the hydrocarbon phase, maintaining the temperatures and/or pressure outside hydrate formation conditions and introducing an antifreeze such as methanol, ethanol, propanol or ethylene glycol. However, the amount of antifreeze required to prevent hydrate blockages is typically between 10% and 20% by weight of the water present in the oil or gas stream. Consequently, several thousand gallons per day of such solvents are usually required. Such quantities present handling, storage, recovery and potential toxicity issues. Moreover, these solvents are difficult to completely recover from the production or transportation stream. From an engineering standpoint, maintaining temperature and/or pressure outside hydrate formation conditions requires design and equipment modifications, such as insulated or jacketed piping. Such modifications are costly to implement and maintain. Furthermore, complex and difficult procedures must be used to ensure that the hydrate blockage is melted safely.
References also show that the effects of gas hydrates can be reduced, in the event that hydrates form, through the presence of compounds that substantially prevent the agglomeration (sometimes referred to as "global formation") of hydrates. For example, surfactants are known to prevent hydrate agglomeration when used at or near the critical micelle concentration (CMC). These surfactants have oleophilic chains having at least 12 carbon atoms.
Consequently, there is a need for a gas hydrate anti-agglomerate useful at low concentration that can be conveniently stored, handled and dissolved in the water phase associated with the produced or transported petroleum fluids. Such an anti-agglomerate would prevent hydrate blockages by inhibiting the agglomeration of gas hydrate crystals in an oil or gas stream.